An internal combustion engine relies upon a lubricating fluid, typically natural or synthetic oil, to reduce friction between moving parts and to remove heat from critical components of the engine. It is well known that the desirable properties of such fluids will degrade with continued operation of the engine, due to chemical changes in the fluid itself as well as the accumulation of contaminants that cannot be successfully filtered out of the fluid.
Many schemes are used to maintain the quality of lubrication in the internal combustion engine of a vehicle. Lubricating oil may be removed and replaced after a predetermined time period, after a predetermined number of operating hours, or after a predetermined number of miles of travel of the vehicle. Such schemes are necessarily conservative to avoid engine damage during worst-case operating conditions, such as high temperature, high speed, high altitude, and dusty environment operation. Such predetermined interval schemes result in unnecessarily frequent oil changes for most normal operating situations.
U.S. Pat. No. 5,749,339 describes a system for automatically replacing a portion of the lubricating oil in an internal combustion engine of a vehicle at a rate influenced by the fuel consumption of the engine. The removed portion of the oil is consumed in the engine by mixing it with the fuel supply for the engine, and fresh oil is added from an on-board supply. U.S. Pat. No. 5,964,318 describes a similar on-board lubricating oil quality management system that controls the amount of oil removed and replaced in response to a continually sensed oil quality parameter. The on-board sensors may be used to determine oil temperature, pressure, dielectric or viscosity values. An on-board controller receives the sensor signal as an input, and it automatically controls an on-board valve for regulating the amount of oil removed from and added to the engine. The controller may also be programmed to consider variables such as the number of starts, run time or distance and fuel usage. Such on-board systems are useful for extending the time period between oil changes for an engine; however, they require expensive on-board diagnostic and fluid systems.
It is common practice in the railroad industry to perform a laboratory oil analysis on a test sample of lubricating oil from a locomotive diesel engine, and to base an oil change/no-change decision on the results of the oil analysis. The oil analysis performed on the test sample is generally more thorough than the type of on-board measurements described in the prior art '318 patent. If the as-measured quality of the lubricating oil is determined to be below a predetermined threshold, the locomotive is scheduled for a complete engine oil change. The quality parameters may include pentane insolubles, viscosity, and total base number (TBN). The quality parameter(s) set point levels used to make this decision are selected to ensure that the oil quality will not drop to an unacceptable level at any time before the next scheduled service outage for the locomotive, typically ninety (90) days hence. Accordingly, it is often necessary to change lubricating oil that is still performing adequately in order to avoid the possibility that it will fall below a quality threshold during the next engine-operating period.